1. Field of the Invention
The present invention relates generally to a wireless mobile communication system and, more particularly, to initial access procedure for a terminal to initially access a base station or a cell, and a method and apparatus for transmitting/receiving common channel information therefore in a mobile communication system supporting a Multiple Input Multiple Output (MIMO) beamforming.
2. Description of the Related Art
Mobile communication systems have evolved into high-speed, high-quality wireless packet data communication systems that provide data and multimedia services beyond those of the early voice-oriented services. Various mobile communication standards, such as, for example, High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), and LTE-Advanced (LTE-A) defined in 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) defined in 3rd Generation Partnership Project-2 (3GPP2), and 802.16 defined in Institute of Electrical and Electronics Engineers (IEEE), have been developed to support the high-speed, high-quality wireless packet data communication services.
In a wireless mobile communication system, a terminal is required to perform an initial access procedure to communicate with a base station. In the initial access procedure, the terminal receives a synchronization signal or Synchronization CHannel (SCH) to acquire downlink synchronization, checks frame timing or a Cell IDentifier (ID), and receives unique system information, base station information, or cell information.
Most communication standards adopt a multi-carrier multiple access technique such as, for example, Orthogonal Frequency Division Multiplexing (Multiple Access) (OFDM(A)) using multiple subcarriers. In a multi-carrier multiple access-based wireless mobile communication system, channel estimation and measurement performance is influenced by the number of symbols and the number of subcarriers to which the reference signal is mapped on the time-frequency resource grid. The channel estimation and measurement performance is also influenced by the power that is allocated for reference signal transmission. Accordingly, by allocating more radio resources (including time, frequency, and power), it is possible to improve the channel estimation and measurement performance, resulting in improved received data symbol demodulation and decoding performance and channel state measurement accuracy.
In a resource-constrained mobile communication system, however, if a radio resource is allocated for transmitting resource signals, the resource amount for data signal transmission is reduced. For this reason, the resource amount for the reference signal transmission is determined by taking the system throughput into account.
Existing 3rd generation mobile communications including LTE, Ultra Mobile Broadband (UMB), and 802.16m operate based on a multi-carrier multiple access scheme, and adopt MIMO with channel sensitive scheduling such as, for example, beamforming and Adaptive Modulation and Coding (AMC), to improve transmission efficiency. Furthermore, many efforts are being made to improve the transmission efficiency with technical enhancements of the MIMO and beamforming techniques. One such effort to improve transmission efficiency is Full-Dimension MIMO (FD-MIMO), which is a technique capable of forming various beams with a few dozen antennas.
FD-MIMO is a technique for forming a narrow and long transmit beam to transmit data using a plurality of antennas so as to send the data to a terminal (or User Equipment (UE)) that far from the base station (or evolved Node B (eNB)) at a low transmit power. The FD-MIMO makes it possible to form various types of beams depending on the number of antennas, and also makes it possible to freely adjust the size, distance, and width of a beam according to the weights applied to the antennas, to a certain extent.
FIG. 1 is a diagram illustrating the concept of the FD-MIMO. In FIG. 1, an eNB 101 uses the FD-MIMO technique, and manages three cells 102, 103, and 104. The eNB 101 is required to provide UEs with a data transmission/reception service within the coverage area of the cell 102. The eNB 101 is required to guarantee a satisfactory data transmission to UE 110 located at a cell edge 106. Using the FD-MIMO technique, it is possible to form a narrow beam 112, 113 using several antennas and to concentrate the power within the beam 111, so as to transmit data to the UE 110 at relatively low transmit power, as denoted by reference number 111. Specifically, when it is possible to form a narrow beam with the FD-MIMO, it is also possible to reduce the transmit power for transmitting the same data as compared to the legacy method.
Based on the low transmit power characteristic of the FD-MIMO, the eNB 101 is capable of maintaining the transmit power at a low level within the cell 102. If the eNB is able to maintain the low transmit power level, it is possible to reduce the power range supported by the power amplifier installed in the eNB 101 and, as a consequence, significantly reduce the cost of the power amplifier. Since the cost of the power amplifier is an important factor in determining the eNB installation cost, the FD-MIMO is advantageous in view of the entire system implementation cost. Furthermore, the FD-MIMO is advantageous in that the reduced average power consumption makes it possible to contribute the environment-friendly Green Communication initiative.
In FIG. 1, if the conventional method using no FD-MIMO beamforming is applied, the data transmission coverage is restricted to the area as denoted by reference number 105 within the area as denoted by reference number 102.
Even when the eNB 101 transmits data to the UE 110 located at the cell edge at a relatively low transmit power, the data can be delivered to the UE 110 with the beamforming gain. In the case of the data broadcast within a cell, however, it is difficult for the eNB 101 to generate the signal covering the entire cell at the power level determined in consideration of the FD-MIMO power gain. For example, in order to generate the signal which UEs 110, 121, and 122 can receive within the cell 102, it is necessary to allocate a transmit power strong enough to cover the entire cell, which the eNB 101 is not able to support.
There are many signals that should be broadcast within the cell, e.g. common channel information such as an SCH necessary for acquiring synchronization between the UE and the eNB and a Broadcast CHannel (BCH) in which the eNB broadcasts the cell information.